Periscope

  A periscope is an optical instrument which enables a person to obtain a view otherwise impossible to see. Periscopes allow a submarine crew to survey objects on the surface of the water. A simple periscope can be made by mounting two mirrors on an angle within a tube or a narrow box. A person can look around a corner or over a fence with this homemade periscope.
  A submarine periscope consists of a long, stainless steel or bronze tube. The optical lenses and prisms are sealed at the top by a glass window so they are watertight. When the periscope is raised above the surface of the water, light enters through the window. The light striking a right-angle prism at the top, is totally reflected downward through several lenses to a second prism or a mirror. At this level the light is again totally re-flected to the eyepiece, and thus to the observer.
  Periscopes are also used for other military purposes. Warships and gun turrets may contain range-finding periscopes designed to protect the operator from enemy fire. Tanks use periscopes, as do foot soldiers in trenches.
  Periscopes are employed to observe radioactive materials. This is one way scientists can see over or through protective walls.